The invention relates to electrochemical cells having a metal current collector such as the steel casing of an alkaline cell. The invention relates to metal current collectors, particularly the steel casing of an alkaline cell, which may be coated with a layer of cobalt and a layer of carbon over the cobalt to extend the useful service life of the cell.
Conventional alkaline electrochemical cells are formed of a cylindrical casing. The casing is initially formed with an enlarged open end. After the cell contents are supplied, an end cap with insulating plug is inserted into the open end. The cell is closed by crimping the casing edge over an edge of the insulating plug and radially compressing the casing around the insulating plug to provide a tight seal. A portion of the cell casing forms the positive terminal.
The cell contents of a primary alkaline cell typically contain zinc anode active material, alkaline electrolyte, a manganese dioxide cathode active material, and an electrolyte permeable separator film, typically of cellulose. The anode active material comprises zinc particles admixed with zinc oxide and conventional gelling agents, such as carboxymethylcellulose or acrylic acid copolymers, and electrolyte. The gelling agent holds the zinc particles in place and in contact with each other. A conductive metal nail, known as the anode current collector, is typically inserted into the anode active material. The alkaline electrolyte is typically an aqueous solution of potassium hydroxide, but other alkali solutions of sodium or lithium hydroxide may also be employed. The cathode material is typically of manganese dioxide and may include small amounts of carbon or graphite to increase conductivity.
Canadian patent 1263697 discloses that the inside surface of a steel casing for an alkaline cell, such as a nickel plated steel casing, may be coated with a carbon lacquer coating which dries to a hard carbon coating to provide a low resistance current path between the MnO2 cathode active material and the cell casing.
U.S. Pat. No. 4,760,002 discloses that the inside surface of the steel casing of an alkaline cell, such as a nickel plated steel casing may be coated with a layer of cobalt or cobalt containing compounds such as Ni/Co or Ni/Co/Fe alloy. The reference teaches that the cobalt may be deposited on the steel casing by reducing a cobalt salt solution, preferably by electrochemically reducing cobalt ions in solution. The resulting cobalt coating reduces the electrical contact resistance between the MnO2 cathode active material and cell casing.
Since commercial cell sizes are fixed, it has been desirable to, attempt to increase the capacity, i.e., the useful service life of the cell by increasing the surface area of the electrode active material and by packing greater amounts of the active material into the cell. This approach has practical limitations. If the active material is packed too densely into the cell this can reduce the rate of electrochemical reaction during discharge, in turn reducing service life. Other deleterious effects such as polarization can occur, particularly at high current drain (high power applications). Polarization limits the mobility of ions within the electrode active material and within the electrolyte, which in turn reduces service life. The contact resistance between the MnO2 cathode active material and the cell casing of an alkaline cell also reduces service life. Such contact resistance losses typically increases, particularly as the cell is discharged during high power applications (between about 0.5 and 1 watt). Modern electronic devices such as cellular phones, digital cameras and toys, flash units, remote control toys, camcorders and high intensity lamps are examples of such high power applications. Thus, it is desirable to provide a way of reliably increasing the useful service life of conventional primary alkaline cells particularly for cells to be used in high power applications, without noticeably increasing polarization effects or otherwise adversely affecting cell performance.
Accordingly it is desirable to extend the useful service life of electrochemical cells, particularly alkaline cells intended for high power applications.
It is desirable to find coatings for the inside surface of a conventional steel casing for an alkaline cell which reduces the contact resistance and increases cell life, particularly during high power applications.
The present invention is directed to a coating on the inside surface of the steel casing of an electrochemical cell, particularly an alkaline cell, to improve the cell""s performance. In accordance with the invention a metal current collector which may be in the form of a metal ""surface such as the inside surface of a steel casing for the cell, preferably preplated with nickel, is plated with a layer of cobalt or cobalt alloy, and then subsequently coated with a layer of carbon. The tandem cobalt and carbon coatings of the invention may be applied to other metal current collectors, preferably those which have been preplated with nickel. Such other current collectors, for example, may be in the form of a wire mesh (woven or unwoven), expanded metal foil, metal sheets, or metallic fibers, or metallic particles which are preferably preplated with nickel. (The current collector is a metal surface in physical contact with an electrode (anode or cathode) and is electrically connected to a cell terminal. The current collector functions to provide a direct path of current flow from the electrode to a cell terminal.)
In an alkaline cell the cathode current collector is preferably the cell""s steel casing. In accordance with the invention a cobalt layer may be electrodeposited onto the inside surface of the cell""s steel casing. Preferably the steel casing has been preplated with nickel. Cobalt alloys such as Ni/Co or Ni/Co/Fe alloy may be electrodeposited in place of or in conjunction with cobalt. The cobalt plating is preferably subjected to a final heat treatment of between about 580xc2x0 C. to 710xc2x0 C. A carbon coating may be applied over the cobalt plating as a carbon/solvent coating. The carbon coating preferably comprises particulate carbon, solvent carrier and binder. The carbon coating upon evaporation of solvent carrier forms a hard carbon coating over the cobalt. The particulate carbon is desirably in the form of particulate carbon, graphite or acetylene black, preferably particulate graphite. The cobalt is deposited to a thickness preferably between about 0.01xc3x9710xe2x88x926 and 1.0xc3x9710xe2x88x926 meters. The dry carbon coating on the surface of the cobalt layer desirably has a thickness of between about 0.0025 and 0.076 mm.
It has been determined that if the inside surface of an alkaline cell""s steel casing, preferably a nickel plated steel casing, is plated with cobalt and subsequently coated with carbon a significant improvement in the cell""s service life is realized when the cell is discharged for high power application. Surprisingly, under high power application the percent improvement in service life is greater than if the nickel plated steel casing were coated with the cobalt or carbon coating alone.
The invention extends to electrochemical cells other than alkaline cells provided such cells have a steel casing, preferably a nickel plated steel casing, or other metallic current collector which comes into contact with at least one of the anode or cathode. The invention is particularly applicable to such cells wherein oxidation or corrosion of the surface of the current collector is a problem impacting on the cell""s performance. In such cells the tandem cobalt and carbon coatings on the inside surface of the nickel plated steel casing can reduce the chance of oxidation or corrosion and extend the cell""s service life.